1. Field of the invention
The present invention generally relates to a cold accumulation type refrigerating machine which may also be referred to as the regenerative refrigerator and which is equipped with a regenerative heat exchanger also known as the cold accumulator or holdover. More particularly, the invention is concerned with the cold accumulation type refrigerating machine which exhibits high refrigeration performance and efficiency owing to enhanced heat exchange efficiency of the cold accumulator or holdover.
2. Description of the Related Art
For having a better understanding of the invention, the technical background thereof will first be described in some detail. FIG. 18 is a diagram showing schematically a structure of the cold accumulation type refrigerating machine (regenerative refrigerator) which is known heretofore in the art, as is disclosed, for example, in Japanese Patent Publication No. 10255/1971. Referring to the figure, the refrigerating machine includes as major components an electric motor 21 serving as a prime mover of the refrigerator, a gas compressor 4 and a gas expansion/refrigeration assembly which is comprised of a cylinder 13 and a displacement member 14. Connected operatively to a rotatable output shaft of the motor 21 is a sort of crank mechanism for translating the rotation of the motor 21 into a linear reciprocative motion of the displacement member 14 within the cylinder 13. To this end, the crank mechanism is composed of a rotatable disk 10A operatively coupled to the output shaft of the motor 21, a rotatable cam roll 10 mounted on the disk 10A at a radial position eccentric to the center of the disk 10A and rotatable within a rectangular frame 10C which is connected to the displacement member 14 through a piston rod 12. It will readily be appreciated that the rotation output of the motor 21 is translated into the linear reciprocative motion of the displacement member 14 within the cylinder 13 through the crank mechanism mentioned above. The displacement member 14 incorporates and carries a cold accumulator (holdover) 15, i.e., a sort of heat exchanger, which may be comprised of a mesh of phosphor bronze or lead balls for storing or holding over the cold. A hermetically closed top chamber 16 is defined between the inner surface of the top wall of the cylinder 13 and the top surface of the displacement member 14, while a hermetically closed bottom chamber 17 which serves as a gas expansion chamber is defined between the inner surface of a bottom wall 20 of the cylinder 13 and the bottom surface of the displacement member 14. The closed top chamber 16 and the bottom expansion chamber 17 are isolated from each other by means of a seal member 18 disposed fluid-tightly between the inner wall of the cylinder 13 and a peripheral surface of the displacement member 14 in the vicinity of the top end thereof. This seal 18 is secured to the displacement member 14 so as to be movable together with the latter. The closed top chamber 16 is in fluid communication with the cold accumulator 15 through a passage 14a, and the expansion chamber 17 is also communicated to the cold accumulator 15 via a passage 14b. Thus, it can be said that the top and bottom chambers 16 and 17 are in fluid communication with each other through the passage 14a, the cold accumulator 15 and the passage 14b. It is further to be mentioned that a seal member 19 is disposed fluid-tightly around the piston rod 12 so as to hermetically close the interior of the cylinder 13 from the atmosphere. Extending from the top chamber 16 of the expansion/refrigeration assembly is a pipe 5 which is bifurcated into a branch pipe 5A connected to an inlet port of the gas compressor 4 and a branch pipe 5B connected to a discharge port of the compressor 4. A gas suction (feed) valve 7 is installed in the branch pipe 5B while a gas discharge (feedback) valve 9 is installed in the branch pipe 5A. Closing and opening of these valves 7 and 9 are controlled under respective predetermined timings, as described hereinafter, by means of actuator cams 6 and 8 which are also operatively coupled to the output shaft of the motor 21. The bottom wall 20 of the cylinder 13 constitutes a heat conducting portion from which the cold is emitted.
Next, description will be directed to the operation of the refrigerating machine of the structure described above. A gas such as a helium gas discharged at a high pressure from the compressor 4 after having undergone compression therein flows into the hermetically closed top chamber 16 defined within the cylinder 13 via the pipes 5B and 5 when the suction valve 7 is opened by the actuator cam 6. The gas within the top chamber 16 is introduced into the cold accumulator 15 through the gas passage 14a to be cooled by the cold stored in the cold accumulator 15 in the preceding cycle. The gas thus cooled flows into the bottom or expansion chamber 17 through the gas passage 14b. At this time, the seal 19 prevents the gas from leaking exteriorly of the cylinder 13. Further, because of the presence of the seal 18, the gas is inhibited from flowing into the top chamber 16 from the bottom chamber 17 through the gap formed between the cylinder 13 and the displacement member 14. Upon reaching the closed bottom chamber 17, the gas is expanded to generate cold since the displacement member 14 moves upwardly, as a result of which articles to be cooled (not shown) are refrigerated through the heat conducting wall 20.
The gas expanded and depressurized within the bottom expansion chamber 17 is caused to pass through the gas passage 14b and flow again through the cold accumulator 15 in the backward direction as the displacement member 14 moves downwardly, as a result of which the cold accumulator 15 is cooled by the gas through heat exchange therewith, whereby the cold is stored in the cold accumulator 15. The gas heated as a result of the regenerative heat exchange mentioned above then flows into the closed top chamber 16 via the gas passage 14a. At that time point, the discharge valve 9 interlocked with the actuator cam 8 is opened with the suction valve 7 being closed. Consequently, the gas within the closed top chamber 16 is fed back to the compressor 4 to be compressed again. The operation described so far constitutes one cycle of refrigerating operation, which is successively repeated.
Next, description will be made of the correspondence relation among the position of the displacement member 14, the open/close timings of the valves 7 and 9 and change in the pressure within the bottom expansion chamber 17 by reference to FIG. 19 along with FIG. 18.
Referring to FIG. 18, the rotation output of the motor 21 is translated into the reciprocative motion of the displacement member 14 through the crank mechanism, as described previously, whereby the displacement member 14 is caused to repetitively move upwardly and downwardly within the cylinder 13, as is illustrated in FIG. 19(a). Parenthetically, in FIG. 19, phase angles are taken along the abscissa such that a phase angle is 0.degree. when the displacement member 14 is at the bottom dead center (BDC) while the phase angle assumes 180.degree. at the top dead center (TDC) of the displacement member 14, and one cycle of operation is completed at a phase angle of 360.degree.. It should further be mentioned that operations of the actuator cams 6 and 8 are interlinked to the rotation of the output shaft of the motor 21 such that the suction valve 7 is opened at a phase angle of -30.degree. and closed at a phase angle of 120.degree., as shown in FIG. 19(b), while the discharge valve 9 is opened at a phase angle of 150.degree. and closed at 330.degree., as illustrated at in FIG. 19(c). The pressure within the closed bottom chamber 17 changes in response to the open/close operation of the suction valve 7 and the discharge valve 9 as well as the displacement of the piston-like member 14. More specifically, from the moment when the suction valve 7 is opened at the phase angle of -30.degree., the pressure within the closed bottom chamber 17 increases while it decreases from the moment when the discharge valve 9 is opened at the phase angle of 150.degree..
In the meanwhile, the rotation speed of the output shaft of the motor 21 remains constant or uniform throughout the operation cycle. Accordingly, the time taken for the phase to advance by 1.degree. is equal to a quotient obtained by dividing the time taken for the one operation cycle by 360. Assuming, by way of example, that the time of one cycle is one second, the displacement member 14 is forced to move in such a manner as illustrated in FIG. 20 in which the origin represents the bottom dead center. On the other hand, the pressure within the bottom expansion chamber 17 changes in such a manner as illustrated in FIG. 21.
In the cold accumulation type refrigerating machine known heretofore and implemented in the Structure described above, the rotation speed (rpm) of the motor 21 is maintained constant. Consequently, the mass flow rate of the gas passing through the regenerative heat exchanger or cold accumulator 15 becomes maximum at the moment when the discharge valve 9 is opened. FIG. 22 is a view for graphically illustrating the mass flow rate of the gas entering the closed bottom chamber 17 after passing through the cold accumulator 15 when the temperature of the heat conducting wall 20 is at 50 K. Parenthetically, the graph of FIG. 22 is plotted on the assumption that the gas flow rate o takes a positive value when the gas enters the bottom expansion chamber 17. The efficiency of the cold accumulator or regenerative heat exchanger 15 becomes lowest at the moment when the discharge valve 9 is opened (corresponding to the time point of 0.5 sec. in FIG. 22). In this conjunction, it is noted that the cold accumulator or regenerative heat exchanger 15 is ordinarily designed on the basis of a mean gas flow rate over one cycle. Consequently, when the actual gas flow rate is deviated significantly from the designed or indicated mean gas flow rate, it becomes impossible to realize the indicated efficiency of the cold accumulator 15, which in turn means that the refrigeration efficiency of the refrigerating machine is eventually degraded, giving rise to a problem.
FIG. 23 is a view graphically illustrating a change in the mass flow rate of the coolant gas flowing into the bottom expansion chamber 17 via the cold accumulator 15 when the temperature of the heat conducting wall 20 is at 4.2 K. In this case, the coolant or helium gas may be regarded as being essentially not in the compressed state. Consequently, at the moment when the discharge valve 9 is opened, the gas flow rate does not undergo any appreciable change. Rather, the gas flow rate increases as the speed of the displacement member 14 becomes higher. Moreover, it is noted that the maximum flow rate of the gas flowing into the bottom expansion chamber 17 is high as compared with the gas flow expelled from that chamber 17. In any case, the gas flow rate deviates from the mean value, providing obstacle in realizing the indicated or designed efficiency of the heat exchanger constituting the cold accumulator, whereby the efficiency of the refrigerating machine is also degraded, to a disadvantage.